Re: [Ifeffit] Transmission EXAFS sample

22 Nov 2010


      On 21.11.2010 19:00, ifeffit-request@millenia.cars.aps.anl.gov wrote:
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Today's Topics:
1. Re: Transmission EXAFS sample (Jatinkumar Rana)
    2. Re: Transmission EXAFS sample (Scott Calvin)
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Message: 1
Date: Sun, 21 Nov 2010 11:44:25 +0100
From: Jatinkumar Rana
To:ifeffit@millenia.cars.aps.anl.gov
Subject: Re: [Ifeffit] Transmission EXAFS sample
Message-ID:<4CE8F809.3040208@helmholtz-berlin.de>
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On 20.11.2010 19:00,ifeffit-request@millenia.cars.aps.anl.gov  wrote:
...
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When replying, please edit your Subject line so it is more specific
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Today's Topics:
1. Re: Transmission EXAFS sample (Scott Calvin)
     2. Re: Transmission EXAFS sample (Frenkel, Anatoly)
----------------------------------------------------------------------
Message: 1
Date: Fri, 19 Nov 2010 10:30:37 -0800
From: Scott Calvin
To: XAFS Analysis using Ifeffit
Subject: Re: [Ifeffit] Transmission EXAFS sample
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Hi Jatin,
Matt covered most of what I would say, but I'll add a few comments of
my own.
I'm not sure how you arrived at the conclusion that you have only a
few percent of what you need--you must be assuming a sample area
somehow. I have frequently made transmission measurements on samples
where I only had a few milligrams available. Generally, I did it by
spreading it on a layer of tape as well as I could and then folding
the tape over and over again--sometimes to make as many as 16 layers.
(Of course, that many layers is not advisable if you're below 6 keV or
so, as the absorption of the tape itself would kill the signal). Even
if there are lots of pinholes because you can't cover the tape
effectively, 16 layers from folding will make them cancel out fairly
well. I can then narrow the beam a bit to match the size of my sample.
Flux isn't really the issue here, so I don't even need a focussed
beamline--I can just narrow the slits.
Two other tips:
1) Realize that even with a tiny amount of sample that much of it
won't end up on the tape. The process of brushing on tape is designed
to separate the small grains from the big ones, with only the small
ones ending up on tape. Allow that to happen!
2) You can sometimes get a second piece of tape to have some sample on
it by putting it sticky side down on your mortar and peeling it back.
A thin layer of dust from the sample will stick to the tape, and give
you a little more absorption and a bit more of a uniform distribution.
If you stack that with the primary piece of tape and then fold a few
times, you may end up in pretty good shape, as long as you're not
operating at a low enough energy so that all the layers of tape are a
problem..
This procedure doesn't give me the best data I've ever seen, but it's
often not bad.
--Scott Calvin
Sarah Lawrence College
On Nov 19, 2010, at 8:13 AM, Matt Newville wrote:
...
Dear Jatin,
The idea that the optimum absorption length (mu*t) for transmission
experiments is 2.3 assumes that the errors in the measurement are due
to counting statistics of the x-rays.  For any synchrotron experiment,
the number of x-rays in the transmission chamber is high enough that
the noise from counting statistics is rarely significant.  This means
that using a value of 2.3 is really not that important.
The more important issues are
   a) having a uniform sample.
   b) not having (mu*t) so high that higher-order harmonics dominate
the transmission measurement.
For transmission measurements, it's difficult to overstate the
importance of a uniform sample.  For an ideal thickness, I would say
that the better rules of thumb than mu*t = 2.3 are to aim for an edge
step of 0.1 to 1.0, and a total absorption less than 3.0.
If you only have enough material for an edge step as low as 0.02 (as
you imply), then measuring in fluorescence or electron emission is
probably a better choice.  Such a sample won't be severely affected by
"self-absorption" (or "over absorption" to use the term this mailing
list prefers) in the fluorescence measurement.  I would recommend
simultaneously measuring transmission and florescence for such a
sample.
My concern about a very thin sample is uniformity.  Specifically, is
the grain size really well below mu/0.02 so that a collection of
particles can give a uniform thickness?  Since you didn't give any
details of the system, it's hard to guess.
Is it feasible to pack that material into a smaller area so that the
thickness is increased and use a smaller x-ray beam?
...
-- Can my sample be only few percentage of the "actual amount" (i.e.
calculated based on above fact) required, and still i can perform
transmission EXAFS ? How would this affect my data ? (I guess, it
will be
heavily dominated by noise)
I would guess that a sample with mu*t of 0.02 would be dominated by
pinholes.
...
-- What if, i have required amount of sample but since material's
density is
so high that it yields only small volume of powder (for a given
weight),
that it can not be covered up on multiple layers of Kapton tape to
ensure
pinhole-free sample ?
If you cannot get the grain size small enough to have many overlapping
grains in the sample, the sample won't be uniform enough for good
transmission data.  The techniques of using multiple layers of mixing
with a low-Z binder don't solve this problem.  These do help to make a
uniform collection of overlapping grains, but don't make the grains
smaller.
I would recommend trying to increase the thickness at the expense of
cross-sectional area, and/or measuring in both transmission and
fluorescence.
Hope that helps,
--Matt
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------------------------------
Message: 2
Date: Fri, 19 Nov 2010 23:21:43 -0500
From: "Frenkel, Anatoly"
To: "XAFS Analysis using Ifeffit"
Subject: Re: [Ifeffit] Transmission EXAFS sample
Message-ID:<69bc35f1-e92c-426c-8786-e2a5c881fae8@blur>
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  DelSp="Yes"
That's probably how they discovered graphene, by trying to make exafs
sample.
Anatoly
Sent from my mobile phone, please forgive typos.
-----Original message-----
From: Scott Calvin
To: XAFS Analysis using Ifeffit
Sent: Fri, Nov 19, 2010 18:30:37 GMT+00:00
Subject: Re: [Ifeffit] Transmission EXAFS sample